The present invention generally relates to memory devices for use with computers and other processing apparatuses. More particularly, this invention relates to non-volatile mass storage devices configured for use as a cache for a system comprising conventional hard disk drive storage.
Compared to solid-state drives, conventional hard disk drives are still the media of choice with respect to cost per Byte. In addition, hard disk drives have practically unlimited data retention whereas all present solid-state drives are non-volatile storage devices, however, data retention is limited to a few months with current NAND flash geometries, and further declining with future process nodes. The price disparity that is still prevailing even after NAND flash production has exceeded that of all other memory devices combined, along with the lack of long term data retention has redefined the primary role of solid-state drives from a replacement of hard disk drives to become a complementing storage device, primarily to service random access patterns of relatively small but frequently accessed data sets.
This type of device is generally considered a cache, and management of data between the hard disk drive as main storage and the solid-state drives as cache is done by one or the other form of hierarchical storage management (HSM). In most cases, HSM uses access frequency of logical block addresses or similar algorithms to determine which data are kept on the faster but smaller solid-state drives and which data are committed to the slower but larger hard disk drive.
Most current physical implementations use a dedicated solid-state drives on its own serial advanced technology attachment (SATA) or other system interface port in addition to the port used to connect the hard disk drive. Both drives use standard SATA cabling for power delivery and exchange of data with the host system. Alternatively, integrated hybrid devices combining both rotatable media and solid state technology are available in the form of peripheral component interconnect express (PCIe) expansion cards. These devices are hard-wired internally, meaning that they don't require extra cables. However, they follow the same logic configuration in that a host bus adapter is integrated onto the card and each storage device is coupled to its own dedicated port.
According to the above, current solid-state drives-based cache solutions have at least two shortcomings. First, in the case of conventional SATA drives, the installation of a cache drive requires additional cabling and mounting of hardware. Secondly, both configurations share the drawback of shuffling the data through several steps, for example, from the hard disk drive to the host bus adapter and via a direct memory access (DMA) channel to the system memory and then finally back to the solid-state drives. In short, the physical implementation of existing cache solid-state drives is cumbersome and the data flow is far from optimal, adding the potential problem of bus contention and increased latencies because of interrupts on several levels of the system. Both issues underscore the need to develop more streamlined and more flexible solutions.
In view of the above, it can be appreciated that there are certain problems, shortcomings or disadvantages associated with the prior art, and that it would be desirable if improved cache solid-state drives systems were available utilizing more efficient data management methods.